It is generally known in the field of roller bearings that needle bushings are the form of needle bearing that have the smallest radial overall height and permit especially space-saving and easy-to-install bearings with high radial load capacity. Such needle bearings are known, for example, from the applicant's “Roller Bearings” catalog, October 2008 edition on pages 681 to 697, and are used as an economical alternative to cylindrical roller bearings, including in the bearings of countershafts in manual transmissions or dual-clutch transmissions in motor vehicles. However, because these needle bushings have a thin-walled outer ring that is formed from sheet steel without cutting and is pressed into a bore in a transmission housing consisting of a light metal, a loose seating of the needle bushing occurs when the temperature rises during motor vehicle operation on account of the different coefficients of thermal expansion. Aluminum and magnesium, the customary materials for a transmission housing, have coefficients of thermal expansion more than twice as high as that of the steel used for the outer ring, so that the bore in the transmission housing provided as the bearing seat enlarges faster than the outer ring of the needle bushing. The result is a rotation of the outer ring along with the supported shaft, which results in increased wear and inaccuracies in the bearing seat, and ultimately to failure of the needle bushing and damage to the transmission.
Attempts were made in the prior art to remedy these disadvantages by means of a needle bushing that likewise consists of a thin-walled outer ring that is formed without cutting and can be inserted into a bore in a housing but has on only one axial side a thin-walled outer ring with a radially inward-pointing fixed rim, and also consists of a needle roller and cage assembly that is inserted into the outer ring and that is composed of a needle cage and a plurality of bearing needles guided with uniform circumferential spacing therein and rolling on the inner circumferential surface of the outer ring and on the outer circumferential surface of the shaft to be supported or on a separate inner ring. On its other axial side, multiple uniformly spaced retaining projections extending radially away from the outer ring have been formed on the outer ring; the retaining projections were inserted in corresponding grooves in the housing and thus constituted an anti-rotation feature of the outer ring. For manufacturing reasons, however, due to these retaining projections it was no longer possible to form the radially inward-facing flanged rim that is otherwise customary on this axial side of the outer ring and that holds the needle roller and cage assembly in the outer ring.
Therefore, multiple uniformly spaced retaining claws that engaged around the fixed rim of the outer ring were formed on the axial side of the needle cage facing the fixed rim in order to prevent the needle roller and cage assembly from moving out the other axial side of the outer ring.
However, in practice it has been shown that such retaining claws on the needle cage are unsuitable for retaining the needle cage in the outer ring over the full service life, since they tend to break off when axial forces arise in bearing operation. Consequently, DE 101 23 965 A1 proposed to implement retention of the needle cage in the outer ring by means of an additional annular thrust washer that is designed with special retaining tabs on its outer circumference corresponding to the outer diameter of the outer ring; these retaining tabs are present in the same number and arrangement as the retaining projections on the outer ring, and are placed on the open axial side of the outer ring to form a second radially inward-facing rim. The thrust washer is then attached to the retaining projections of the outer ring using the retaining tabs, and is pressed into the bearing seat together with the needle bushing.
However, the formation of a second rim on the needle bushing with an additional thrust washer of this nature has also proven to be disadvantageous, since additional costs arise in its production and assembly. Moreover, it has been shown that the bending of the radial retaining projections on the outer ring gives rise to inhomogeneities in the inner race for the rolling elements formed by the inner circumferential surface of the outer ring, since these retaining projections act as stiffeners on the inner race under heavy load. The regions of the race located directly adjacent to the projections are then considerably more elastic again, resulting in local stress peaks and uneven load distribution in the race due to the rolling elements that roll over these regions. In like manner, the bent retaining projections result in geometric deviations that extend into the race for the rolling elements and thereby adversely affect the load capacity of the needle bushing.